Method and device for determining several parameters of a seated person

ABSTRACT

The invention concerns a method for determining several parameters of a seated person, comprising the following steps: subdividing the seat surface ( 4 ) into at least two sections, determining the barycenter position of the active weight in each section, and evaluating said parameters of said person based on said determined positions. The evaluation of said parameters can include the evaluation of the size and/or weight of said person, the evaluation of the position of the said person on said seat based on the distribution of the barycenter positions on the seat or the evaluation of the orientation of said person on said seat based on the longitudinal positions of the barycenters of the active weight in each section.

The present invention relates to a method and device for determiningseveral parameters of a person sitting on a seat, such as, for example,the size and/or the weight of the passenger and/or the orientation ofthe passenger on the seat. Such a device is particularly applicable inthe area covering the control of the protection system in motorvehicles.

In order to protect the lives of passengers during a traffic accident,modern vehicles are generally fitted with a protection system comprisingseveral airbags and seat belt pre-tensioners, which are used to absorbthe energy of a passenger released during the collision due to theaccident. It is clear that such systems are even more effective whenthey are better adapted to the specific requirements of each passenger,i.e. to the weight and/or the size of the passenger. That is whymicroprocessor-controlled protection systems have been designed whichprovide several operational modes, for example allowing an adaptation ofthe instant at which airbags are deployed and their volume, of theinstant at which safety belts are released after the collision, etc, asa function of the stature of the passenger and the orientation of thepassenger on the seat.

In order to enable the control microprocessor to select the optimumoperational mode for a given passenger, it is therefore necessary tohave available a device for detecting the stature of the passenger whichdetermines the size and/or the weight and/or the orientation of thepassenger and which indicates this to the control circuit of theprotection system. For this purpose, the U.S. Pat. No. 5,232,243describes a device for detecting the weight of a passenger whichcomprises several individual force sensors arranged in a matrix array inthe vehicle seat cushion. The force sensors have an electric resistancethat varies with the applied force and are known by the abbreviation FSR(force sensing resistor). The resistance of each sensor is measuredindividually and, by adding the forces corresponding to the values ofthese resistances, an indication is obtained of the total force exerted,i.e. of the weight of the passenger.

However, the total weight of a passenger does not act solely on thesurface of the seat, since part of the weight is supported by thepassenger's legs, which rest on the bottom of the vehicle, and anotherpart rests on the back of the seat. In addition, the ratios between thevarious parts vary considerably with the passenger's position on theseat, which causes the total force measured by the individual forcesensors not to correspond to the real weight of the passenger but toexperience very large variations depending on the passenger's posture onthe seat.

Moreover, because of variations with temperature of the characteristicsof the seat padding, the individual forces measured by the differentforce sensors depend greatly on the ambient temperature in the vehicle.In fact, at very low temperatures, foam padding for example becomes veryhard, causing the forces measured by the sensors to be less than thereal forces. At very high temperatures, on the other hand, foam paddingexpands and exerts an additional pressure on the sensors, so that theforces measured by the sensors are greater than the forces actuallyexerted. It follows from this that the device for detecting the weightof a passenger, as described in the above-mentioned document, cannotreally satisfy the requirements of a modern protection system whoseoperation must to a large extent be independent of the ambientconditions.

The objective of the present invention is therefore to propose a devicefor determining several parameters of a person sitting on a seat, theoperation of which is to a large extent independent of the temperatureand of the passenger's posture on the said seat.

In conformity with the invention, this objective is achieved by a devicemaking it possible to determine the size and/or the weight of a personsitting on a seat, which operates according to a principle differentfrom that of existing weight detectors. The method used for determiningthe size and/or the weight of a person sitting on a seat involves thesubdivision of the seat's surface into at least two sections, thedetermination of the position of the centre of gravity of the activeweight in each section, and the evaluation of the size and/or the weightof the said person from the said positions so determined. Measurementsare therefore no longer made of the magnitude of the force exerted bythe passenger on the seat, but measurements are instead made of thepositions at which this force acts. In other words, relative values arenow to be measured instead of absolute values. The positions at whichthe force acts determined in this way are therefore to a large extentindependent of the factors affecting the absolute values of the force,such as the posture of the passenger on the seat and the ambienttemperature. The respective positions of the centres of gravity in thedifferent sections of the seat then make it possible to determine thesize and/or the weight of the person and, for example, the person'sposition and/or orientation on the seat.

By subdividing the seat, for example into a plurality of adjacentsections, and by determining the positions at which the weight acts ineach of these sections, it is possible to determine the total area overwhich the weight is active, i.e. the area of the seat occupied by thepassenger. Moreover, it is easy to determine the position of thepassenger on the seat from the distribution of the different positionsof the centres of gravity, and this makes it possible to assess whetherthe passenger is sitting in the middle of the seat. By comparing thelongitudinal positions of the centres of gravity in different laterallyadjacent sections of the seat, it is possible to determine theorientation of the passenger on the seat, i.e. whether the passenger isfacing the front or a different direction. It should be noted that thedifferent parameters are preferably assessed sequentially using the samedetector.

In a preferred version of the method, the surface of the seat issubdivided into two laterally adjacent sections and the evaluation ofthe size and/or the weight involves determining the distance between thepositions of the two centres of gravity of the weight in the said twosections. The parameter so determined is therefore the lateral distancebetween the position at which the weight acts on the left-hand part ofthe seat and the position at which the weight acts on the right-handpart of the seat, i.e. a distance which is correlated with the statureof the passenger. From this distance, it is thus possible to evaluatethe weight and/or the size of the passenger by using a model of a humanbody based on statistical measurements.

It is true that a method of determination using a model of a human bodycannot provide an exact measurement of the real weight of the seat'soccupant. However, in view of the restricted number (3 for example) ofways in which the airbags or seat belt pre-tensioners in a vehicle canfunction, the requirements for the control device of the protectionsystem as regards the accuracy of the real value of the weight are onlyof secondary importance. It is in fact necessary only to allocate thedifferent passengers to a restricted number of categories as regardsweight and size for the control device to be able to select theappropriate operational mode to be applied. In the example of threeoperational modes for the protection system, three categories of weightshave to cover a total range from, for example, 0 to 100 kg, i.e. eachcategory must cover a range of about 30 kg. Now it is clear that, for aclassification into such broad categories, the results obtained byevaluating weight and/or size using a human model to a great extentsatisfy the requirements of accuracy in the system.

In order to work according to the method described above, a device fordetermining the size and/or the weight of a person sitting on a seattherefore comprises a means for determining the respective positions ofthe centres of gravity of the active weight in at least two differentsections of the said seat and a means of evaluating the size and/or theweight of the said person from the said determined positions. Thepositions of the centres of gravity determined in this way give, forexample, an indication of the total area over which the weight isactive, i.e. the area of the seat which is occupied by the passenger. Toachieve this, the surface of the seat should be subdivided into a largenumber of sections. However, in a preferred execution, the said means ofdetermining the positions of the centres of gravity comprise a means ofdetecting the distance between a first centre of gravity of the weighton a first section of the seat and a second centre of gravity of theweight on a second section of the seat, the two sections of the seatbeing laterally adjacent. In other words, the lateral distance ismeasured between the position at which the weight acts, for example onthe left-hand part of the seat, and the position at which the weightacts on the right-hand part of the seat, i.e. a distance which isrelated to the width of the area of the seat occupied by the passenger.This distance then makes it possible to evaluate the weight and/or thesize in the way described above.

The means for determining the positions of the centres of gravitypreferably comprise a position-defining force detector extending overthe surface of the seat. Such a detector consists, for example, of aplurality of switching elements arranged in a plurality of adjacentsections of the seat. These switching elements are then interconnectedin an n×m matrix array so that they can be individually identified.However, such a detector requires a large number (≧n*m) of connectionswith the outside, i.e. with the control device for the protectionsystem, and inside the control device it requires a sophisticatedelectronic system for the real-time exploitation of the n*m signals fromthe different switching elements.

In an advantageous implementation, the said position-defining forcedetector comprises several active areas in the form of strips, the saidactive areas being positioned on both sides of a line separating thesaid two sections and extending parallel to it. The strip-shaped activeareas then advantageously extend over a major part of the length of theseat's surface, so that a determination of the width of the areaoccupied by passengers is independent of their longitudinal position onthe seat. This implementation on the one hand considerably reduces thenumber of connections of the detector with the outside and on the otherhand enables a less sophisticated electronic system to be used for thereal-time exploitation of the signals from the active areas.

Advantageously, the said force detector comprises force sensors whoseelectric resistance varies with the applied force. These force sensorsare known by the abbreviation FSR (force sensing resistors) and enablethe value of the force applied to the active area to be detecteddirectly. This direct measurement of the applied force thus enables thedevice according to the invention also to operate as a detector of theoccupation of the seat. In other words, below a certain value of theforce measured by the FSRs corresponding to a certain minimum weightacting on the seat, the protection system for the seat in question isnot activated at all. During a collision due to an accident, adetermination of the passenger's weight category is made and theprotection system is activated only if the limiting value of the forceis exceeded.

For safety reasons, the device advantageously comprises a circuit formonitoring the integrity of the conductors. This circuit monitors theintegrity of the conductors, for example when the vehicle starts up, andindicates to the control device of the protection system any breakdownin a connection or a conductor. In the case of such a breakdown whichrisks affecting the correct operation of the detection device, thecontrol device will select a standard operational mode of the protectionsystem which represents a compromise solution for all the weightcategories.

Other special features and characteristics of the invention will emergefrom the detailed description of several advantageous embodimentsdescribed below, as illustrative examples, with reference to theappended drawings. These show:

FIG. 1: a first embodiment of a device for detecting weight and/or sizeaccording to the invention;

FIGS. 2a-2 e: a diagram illustrating the operation of aposition-defining detector;

FIG. 3: a second embodiment of a device for detecting weight and/or sizeaccording to the invention;

FIG. 4: a third embodiment of a device for detecting weight and/or sizeaccording to the invention, enabling the integrity of the conductors tobe monitored;

FIG. 5: another embodiment of a device for detecting weight and/or sizeaccording to the invention, capable also of detecting the longitudinalposition and/or the orientation of the passenger on the seat;

FIGS. 6a & 6 b: a diagram summarising the measurements possible with adetector according to FIG. 5 for different modes of connection of theactive areas;

FIG. 7: an embodiment of the detector of FIG. 5 enabling the integrityof the conductors to be monitored.

FIG. 1 shows a preferred embodiment of a device for detecting weightand/or size 2 which is incorporated in the padding of a seat 4 in avehicle. This is an embodiment with a position-defining force detector 2produced using variable-resistance force detectors 6 of the FSR typewhich are arranged on a flexible support (not shown). These FSR sensors6 are represented in the figure by variable resistors.

An FSR sensor is described for example in the U.S. Pat. No. 4,489,302and consists of two layers, the first of which is formed from asemiconductor element and the second of which has two combs ofinterdigital conductors. At zero force, the two layers of the FSR sensorare separated and the resistance between the two conductors is veryhigh. Under the action of a force, the two conductors are shunted by thesemiconducting layer and the resistance between the two conductorsdecreases as the applied force increases. In another type of FSR sensor,two conductors of any shape are separated by an intercalatedsemiconducting layer. Under the action of a force, the two conductorsand the semiconducting layer are pressed together and the resistancebetween the two conductors decreases as the applied force increases.Such an FSR sensor is described for example in the U.S. Pat. No.4,315,238.

In the embodiment of FIG. 1, several FSR sensors 6 are connected at eachinstant to form several active areas 8 in the shape of strips extendingover a large proportion of the length of the seat's surface. Given thegreat variations possible in the dimensions with which the FSR sensors 6may be manufactured, such a strip-shaped active area 8 may also beformed by a single strip-shaped FSR sensor 6.

The active areas 8 are arranged on both sides of a line of separation 10on the surface of the seat and are located symmetrically on the surface.This line of separation 10 subdivides the surface of the seat into twolaterally adjacent sections 12, 14 and is preferably a line of symmetryon the seat 4. So that the position of the centre of gravity of theweight in each of the sections can be measured, the number of activeareas 8 of the force detector 2 in each of the sections 12, 14 of theseat 4 is greater than or equal to two.

The device in the example shown in FIG. 1 comprises three active areason each section 12, 14 of the seat 4, which are laterally spaced out ina more or less regular manner. The different active areas 8 of a section12, 14 of the seat 4 are supplied with different voltages, i.e. a firstconductor of each FSR sensor of an active area is connected to therespective supply voltage. The supply voltage of the active areas 8increases as they pass from the inside of the seat 4 to the outside. Inother words, the active area 8 ₁ located on the inside of the seat nearthe line of symmetry 10 is connected to a first supply voltage T₁, theactive area 8 ₂ located in the middle of each section 12 or 14 isconnected to a second supply voltage T₂ and the active area 8 ₃ locatedon the outside near the edge of the seat 4 is connected to a thirdsupply voltage T₃, with T₁<T₂<T₃. In order to reduce the number ofexternal connections, the different voltages T₁, T₂, T₃ required tosupply the three active areas 8 ₁, 8 ₂, 8 ₃ in each section 12, 14 arepreferably supplied through a linear resistor 16, 18 or through a chainof resistors connected in series, to the terminals of which is applied apotential difference to create a potential gradient. The differentactive areas 8 ₁, 8 ₂, 8 ₃ are then connected to different voltagesdepending on the position of their connection to the linear resistor 16,18.

Through their second conductors, the FSR sensors 6 are connected to anoutput line 20 or 22 of the detector 2. The circuit produced in this waycorresponds to a linear potentiometer whose slider operates as apotential divider between the terminals of the linear resistor 16, 18.With the resistances of the different active areas 8 ₁, 8 ₂, 8 ₃decreasing as the force with which the active areas are activatedincreases, the voltage at the output 20 or 22 takes on a valuecorresponding to a weighted mean of the three supply voltages T₁, T₂,T₃, the weighting being produced by the relative resistances of theactive areas. In other words, the greater the pressure on an activearea, the more the respective supply voltage contributes to the outputvoltage. It should be noted that such a weighting also takes intoaccount the distribution of the weight over the length, i.e. it takesinto consideration the length over which the different active areas arestressed. In fact, the resistance of an active area decreases as thenumber of its FSR sensors that are stressed increases and hence theactive area in principle carries out an integration of the force overthe area acted on by the force. Thus, the voltage measured at the output20 or 22 gives a direct indication of the position of the centre ofgravity of the weight in the respective section. It is clear that,because the supply voltages increase from the inside to the outside ofthe seat, T₁<T₂<T₃, the voltage at the output 20 or 22 will be greateras the centre of gravity moves further to the outside. In other words,the greater the area occupied by the passenger on the seat, the higherthe voltages at the output lines 20 and 22.

By measuring the voltages at the two output lines 20 and 22, thepositions of the centres of gravity of the weight in the two sections ofthe seat 4 are known and in this way the distance between these twocentres of gravity can easily be calculated. It should be noted that itis also possible to connect the two output lines 20 and 22 in order toadd the two output voltages from the two sections of the seat. Thisgives an output signal which is directly proportional to the distancebetween the two centres of gravity of the weight. This embodimentenables the number of external connections to be reduced. However,information about the distribution of the area occupied over the twosections 12 and 14 is lost.

FIG. 2 is a schematic illustration of the connection of the three activeareas as a linear potentiometer. The starting point is a simple linearpotentiometer produced using an FSR force sensor (FIG. 2a). Such alinear potentiometer circuit is described, for example, in the U.S. Pat.No. 4,810,992. It consists of a linear resistor 24 at the terminals ofwhich different voltages are applied so as to create a potentialgradient. Connectors 26 extending laterally at regular intervals areconnected to the said linear resistor 24. The slider 28 of thepotentiometer is formed by a second conductor in the form of a combwhose teeth extend between the connectors 26. By short-circuiting thetwo connectors 26 and 28 at a certain point, the conductor 28 issubjected to a voltage which varies linearly with the position of theconductor 26 on the linear resistor 24. In order to create severalseparate active areas, the active area 30 of the potentiometer is thendivided into several zones 30 ₁, 30 ₂, 30 ₃ (FIG. 2b). These activezones 30 ₁, 30 ₂, 30 ₃ are lengthened in order to form strip-shapedactive areas extending over almost the whole of the length of the seat(FIG. 2c) and are symbolised by variable resistances connected in series(FIG. 2d). If the possibility of monitoring the integrity of theconductors needs to be available, the said conductors are modified insuch a way that they form loops having external connections (FIG. 2e).

FIG. 3 represents a simplified embodiment of the detector in FIG. 1,which makes possible a further reduction in the number of externalconnections required. The detector 2 consists of no more than a singlelinear resistor 16, to the terminals of which are applied a potentialdifference in order to supply the different active areas 8 ₁, 8 ₂, 8 ₃of the two sections 12 and 14 of the seat 4. Each of the active areas 8₁, 8 ₂, 8 ₃ of the section 14 of the seat 4 is for this purposeconnected to the respective active area 8 ₁, 8 ₂, 8 ₃ of the section 12,so that the two areas 8 ₁ are supplied by the same voltage T₁, the twoareas 8 ₂ are supplied by the same voltage T₂, and the two areas 8 ₃ aresupplied by the same voltage T₃.

In this embodiment, the number of external connections is reduced tofour, namely the two terminals of the linear resistor 16 and the twooutput lines 20 and 22. It is even possible to reduce the number ofconnections necessary to three by connecting the two outputs 20 and 22.However, as mentioned above, this causes the loss of information aboutthe weight distribution over the two sections 12 and 14.

Another embodiment of a device for determining the size and/or theweight according to the invention is represented in FIG. 4. This is anembodiment making it possible to monitor the integrity of the differentconductors. For this purpose, all the conductors connecting thedifferent FSR sensors 6 with each other or with the linear resistor 16are arranged so as to form loops with external connections. In order tolimit the number of such external connections, the linear resistor 16is, for example, subdivided into several discrete resistors 16 ₁, 16 ₂,16 ₃, 16 ₄ placed on both sides of the line of separation of the twosections so as to enable all the active areas 8 ₁, 8 ₂, 8 ₃ of the twosections 12 and 14 to be supplied by a single loop. There is a total ofsix external connections, given that each of the output lines 20, 22 isformed by one loop and has two connections 20, 20′ and 22, 22′.

The integrity of the conductors can be monitored by injecting a signalinto a first connection of each loop and by detecting the signal at thesecond connection. This is preferably achieved through the controldevice of the vehicle protection system. When, on one of the outputconnections of the different loops, the control device does not detectthe signal injected at the other connection, it chooses a standardoperational mode of the protection system representing a compromisesolution for all the weight categories.

It should be noted that, for this embodiment, it is also possible toreduce the number of connections still further at the expense ofinformation about the weight distribution by connecting the two outputlines 20 and 22 and by allowing only the connections 20′ and 22′ toleave the system.

FIG. 5 shows an embodiment of a device 2 for detecting severalparameters of a person sitting on a seat 4, with which it is possible todetect in sequence the lateral positions (in the x direction) and thelongitudinal positions (in the y direction) of the centres of gravity ofthe active weight in the different sections of the seat. Depending onthe operational mode, this device therefore makes it possible to detectboth the weight and/or the size of the person and the longitudinalposition and/or the orientation of the person on the seat.

For this purpose, each active area 8 ₁, 8 ₂, 8 ₃ comprises severalindividual sensors 8 _(1,1), 8 _(1,2), 8 _(1,3), 8 _(1,4) or 8 _(2,1), 8_(2,2), 8 _(2,3), 8 _(2,4) or 8 _(3,1), 8 _(3,2), 8 _(3,3), 8 _(3,4)which are arranged one behind the other in the longitudinal direction ofthe seat and which are interconnected at one of their terminals throughthe intermediary of a linear resistor 32 or a chain of discreteresistors connected in series. At the other terminal, the individualsensors 8 _(1,1), 8 _(1,2), 8 _(1,3), 8 _(1,4) or 8 _(2,1), 8 _(2,2), 8_(2,3), 8 _(2,4) or 8 _(3,1), 8 _(3,2), 8 _(3,3), 8 _(3,4) of eachactive area 8 ₁, 8 ₂, 8 ₃ respectively are interconnected through theintermediary of a conductor 34 ₁, 34 ₂ or 34 ₃ respectively.

In a first operational mode, that for the size/weight determination,different voltages T₁, T₂, T₃ are applied in each section 12, 14 of theseat 4 to the conductors 34 ₁, 34 ₂, 34 ₃ of the different active areas8 ₁, 8 ₂, 8 ₃ of each section 12 or 14 such that T₁<T₂<T₃, and theoutput signal at one of the terminals 36 or 38 of the linear resistor 32is measured. In order to reduce the effects of the resistor 32interconnecting the different individual sensors 8 _(1,1), 8 _(1,2), 8_(1,3), 8 _(1,4) or 8 _(2,1), 8 _(2,2), 8 _(2,3), 8 _(2,4) or 8 _(3,1),8 _(3,2), 8 _(3,3), 8 _(3,4) on the measured voltages, the terminals 36and 38 are connected together so that the resistor 32 is connected in aclosed loop.

The detector 2 connected in this way then functions similarly to thedetector of FIG. 1. The circuit thus produced corresponds to a linearpotentiometer whose slider operates as a voltage divider betweenvoltages T₁, T₂ and T₃. With the resistances of the different activeareas 8 ₁, 8 ₂, 8 ₃ decreasing as the force with which the active areasare activated increases, the voltage at the output 36 or 38 takes on avalue corresponding to a weighted mean of the three supply voltages T₁,T₂, T₃, the weighting being produced by the relative resistances of theactive areas.

It should be noted that, for this operational mode, the three supplyvoltages T₁, T₂, T₃ may either be supplied directly to the conductors 34₁, 34 ₂, 34 ₃ by the control device (not represented) of the system, ormay be supplied by a linear resistor or by a chain of resistorsconnected in series, to the terminals of which a potential difference isapplied so as to create a potential gradient (see FIG. 1). The differentactive areas 8 ₁, 8 ₂, 8 ₃ are then connected to different voltagesdepending on the position of their connection to the linear resistor.

In the second operational mode, that for the detection ofposition/orientation, the individual sensors 8 _(1,1), 8 _(1,2), 8_(1,3), 8 _(1,4) or 8 _(2,1), 8 _(2,2), 8 _(2,3), 8 _(2,4) or 8 _(3,1),8 _(3,2), 8 _(3,3), 8 _(3,4) of each active area are supplied bydifferent voltages, so that the supply voltage increases towards therear of the seat. The different voltages are preferably supplied byapplying a potential difference to the terminals 36 and 38 of theresistor 32. The different individual sensors 8 _(1,1), 8 _(1,2), 8_(1,3), 8 _(1,4) or 8 _(2,1), 8 _(2,2), 8 _(2,3), 8 _(2,4) or 8 _(3,1),8 _(3,2), 8 _(3,3), 8 _(3,4) are then connected to different voltagesdepending on the position of their connection to the linear resistor 32.The positions of the connections of the corresponding individual sensors8 _(1,1), 8 _(2,1), 8 _(3,1) or 8 _(1,2), 8 _(2,2), 8 _(3,2) or 8_(1,3), 8 _(2,3), 8 _(3,3) or 8 _(1,4), 8 _(2,4), 8 _(3,4) of therespective different active areas 8 ₁, 8 ₂, 8 ₃ are advantageously thesame, so that the corresponding individual sensors 8 _(1,1), 8 _(2,1), 8_(3,1) or 8 _(1,2), 8 _(2,2), 8 _(3,2) or 8 _(1,3), 8 _(2,3), 8 _(3,3)or 8 _(1,4), 8 _(2,4), 8 _(3,4) are supplied by the same voltage.

In this operational mode, the output voltages from the different activeareas 8 ₁, 8 ₂, or 8 ₃ on the conductors 34 ₁, 34 ₂, 34 ₃ are thenadvantageously measured. The active areas 8 ₁, 8 ₂, 8 ₃ of each section12 or 14 are therefore connected in a matrix array, i.e. the outputvoltage on each output line 34 ₁, 34 ₂, 34 ₃ of each of the active areas8 ₁, 8 ₂, 8 ₃ is measured separately. The circuit produced in this wayfor each active area 8 ₁, 8 ₂, or 8 ₃ corresponds to a linearpotentiometer whose slider operates as a voltage divider between theterminals of the linear resistor 32. Since the resistances of thedifferent individual sensors 8 _(1,1), 8 _(1,2), 8 _(1,3), 8 _(1,4) or 8_(2,1), 8 _(2,2), 8 _(2,3), 8 _(2,4) or 8 _(3,1), 8 _(3,2), 8 _(3,3), 8_(3,4) decrease as the force with which the sensors are activatedincreases, the voltages on the conductors 34 ₁, 34 ₂, 34 ₃ take onvalues corresponding to a weighted mean of the supply voltages of thedifferent individual sensors 8 _(1,1), 8 _(1,2), 8 _(1,3), 8 _(1,4) or 8_(2,1), 8 _(2,2), 8 _(2,3), 8 _(2,4) or 8 _(3,1), 8 _(3,2), 8 _(3,3), 8_(3,4) of the respective active areas, the weighting being produced bythe relative resistances of the individual sensors. In other words, themore the pressure on an individual sensor, the more its respectivesupply voltage contributes to the output voltage on the respectiveconductor 34 ₁, 34 ₂, 34 ₃.

Since the supply voltages of the individual sensors increase from thefront of the seat to the rear, an output voltage is obtained at eachconductor 34 ₁, 34 ₂, 34 ₃ which becomes higher as the person sits moreto the rear of the seat. These output voltages are thereforerepresentative of the longitudinal positions of the centres of gravityof the active weight in the different strip-shaped active areas 8 ₁, 8₂, 8 ₃. From the distribution of these longitudinal positions on theseat, it is then easy to define the longitudinal position of thepassenger on the seat and the orientation of the passenger on the seat.In effect, very low voltages measured on the conductors of the twosections indicate that the person is sitting more on the front edge ofthe seat 4. In addition, a highly asymmetrical distribution of thepositions on the two sections of the seat makes it possible to deducethat the orientation is not towards the front and, as a result, that acommand should be sent to the vehicle airbag.

By making measurements sequentially according to the two operationalmodes of the detector, it then becomes possible, with a single detector,to measure both the size and/or the weight of the person sitting on theseat and the position and orientation of the person with respect to theseat. Since the switching between the two operational modes may takeplace several times per second, it then become possible to detect allchanges in position of the passenger on the seat almost in real time andhence to adapt the deployment of the airbag.

An alternative position/orientation detection mode consists inconnecting the three conductors 34 ₁, 34 ₂, 34 ₃ together and measuringonly the resultant voltage. This alternative applies mainly when thethree active areas are supplied, in the size/weight detection mode,through the intermediary of a linear resistor as described above andsimilar to the embodiment in FIG. 1. In this case, the terminals of thelinear resistor interconnecting the active areas are connected togetherand the voltage applied to each closed loop is measured. Depending onthe method of connecting the active areas, it is still possible todistinguish two different measurements of position, which arerepresented diagrammatically in FIG. 6.

In the case of a supply to the active areas by means of a linearresistor (FIG. 6a), a measurement is therefore made not of the relativepositions of the centres of gravity of the active weight on each activearea 8 ₁, 8 ₂, 8 ₃ but only of the positions of the centres of gravityof the active weight on each section 12, 14. However, this proceduredoes make it possible to measure the real distance between the twocentres of gravity of the active weight in the two sections by takinginto account their longitudinal positions. By comparison, with thedevices according to FIG. 1, only (x₁−x₂)cos_(α) is measured (x₁ and x₂representing the lateral positions of the centres of gravity G₁ and G₂in the two sections of the seat). Moreover, the difference inlongitudinal positions enables the orientation of the passenger on theseat to be determined, i.e. in this case an orientation in a directionwhich deviates from the front direction by an angle_(α).

In the case of a matrix connection of the different active areas (FIG.6b), in the position/orientation mode, the relative positions of thecentres of gravity G₁ . . . G_(n) of the active weight on each activearea 8 ₁, 8 ₂, 8 ₃ are measured. The distribution of these positions G₁. . . G_(n) with respect to the seat still enables the position andorientation of the passenger on the seat to be defined. This operationalmode has the advantage of being able to detect abnormal situations, suchas one in which a child is sitting only on his hands or possibly when anauxiliary seat presses only on feet at the side. In this case, thedetector detects a pressure only for the outer active areas 8 ₃, theinner active areas 8 ₁ and 8 ₂ giving no signal, and deployment of theairbags can be stopped.

FIG. 7 represents an embodiment of the detector in FIG. 5 enabling theintegrity of the conductors to be monitored. For this purpose, all theconductors connecting the different individual sensors 8 _(1,1), 8_(1,2), 8 _(1,3), 8 _(1,4) or 8 _(2,1), 8 _(2,2), 8 _(2,3), 8 _(2,4) or8 _(3,1), 8 _(3,2), 8 _(3,3), 8 _(3,4) of each section 12, 14 to eachother or to the linear resistor 32 are arranged in such a way as to forma loop which has external connections, for example the terminals 36 and38. This is achieved by a subdivision of the linear resistor 32 intoseveral discrete resistors 32 ₁, 32 ₂, 32 ₃, 32 ₄, 32 ₅ which areconnected to each other by conductors supplying the individual sensors.

The integrity of the conductors can be monitored by injecting a signalinto one of the terminals 36 or 38 of the resistor 32 and by detectingthe signal on the second terminal 38 or 36 respectively. This ispreferably carried out by the control device of the vehicle protectionsystem. When the control device does not detect the injected signal, itselects a standard operational mode of the protection system whichrepresents a compromise solution for all weight categories.

What is claimed is:
 1. Device for determining the size of a personsitting on a seat, comprising a means for determining the respectivepositions of the centres of gravity of the active weight in at least twodifferent sections of the said seat and a means for evaluating the sizeof the said person from the said respective positions so determined. 2.Device according to claim 1, wherein the said means for determining thepositions of the centres of gravity comprises a means for detecting thedistance between a first centre of gravity of the weight on a firstsection of the seat and a second centre of gravity of the weight on asecond section of the seat, the two sections of the seat being laterallyadjacent.
 3. Device according to claim 1, wherein said means fordetermining the position of the centres of gravity comprises aposition-defining force detector which extends over the surface of theseat.
 4. Device according to claim 3, characterised in that the saidposition-defining force detector comprises several strip-shaped activeareas, the said active areas being located on both sides of a line ofseparation of the said two sections and extending parallel to the saidline.
 5. Device according to claim 4, wherein the different active areasof a section of the seat are supplied with different voltages.
 6. Deviceaccording to claim 5, wherein each active area of the said first sectionof the seat and the corresponding active area of the said second sectionof the seat are supplied with the same voltage.
 7. Device according toclaim 4, wherein the active areas of a section are supplied by means ofa potential gradient through several resistors connected in series, sothat the circuitry of the active areas represents a linear potentiometercircuit.
 8. Device according to claim 4, wherein each active areacomprises several individual sensors which are placed in line in thelongitudinal direction of the seat.
 9. Device according to claim 8,wherein the individual sensors of each active area are supplied withdifferent voltages.
 10. Device according to claim 9, wherein thecorresponding individual sensors of the different active areas of asection are supplied with the same voltage.
 11. Device according toclaim 9, wherein the individual sensors of a section are supplied bymeans of a potential gradient through a linear resistor, so that thecircuitry of the individual sensors represents a linear potentiometercircuit.
 12. Device according to claim 3, wherein the said forcedetector comprises force sensors whose electric resistance varies withthe applied force.
 13. Device according to claim 4, wherein said activeareas are connected to connection terminals by means of conductors,wherein said device comprises a circuit for monitoring the integrity ofthe conductors.
 14. Device according to claim 1, wherein the said meansfor determining the positions of the centres of gravity is incorporatedin the cushion of the seat.
 15. Method for determining the size of aperson sitting on a seat, comprising the steps: subdivide the surface ofthe seat into at least two sections, determine the respective positionof the centre of gravity of the active weight in each section, andevaluate the size of the said person from the said respective positionsso determined.
 16. Method according to claim 15, wherein the surface ofthe seat is subdivided into two laterally adjacent sections, and whereinthe evaluation of the size comprises the determination of the distancebetween the positions of the two centres of gravity of the weight in thesaid two sections.
 17. Method according to claim 15, comprising theadditional step of evaluating the position of the said person on thesaid seat from the distribution of the positions of the centres ofgravity on the seat.
 18. Method according to claim 15, comprising theadditional step of evaluating the orientation of the said person on thesaid seat from the longitudinal positions of the centres of gravity ofthe active weight in each section.
 19. Method according to one of claim17 or 18, wherein the evaluation of the different parameters is carriedout sequentially.
 20. Use of a device according to claim 1 in thecontrol of an airbag of a motor vehicle.
 21. Device according to claim9, wherein the individual sensors of a section are supplied by means ofa potential gradient through several resistors connected in series, sothat the circuitry of the individual sensors represents a linearpotentiometer circuit.
 22. Device according to claim 8, wherein saidindividual sensors are connected to connection terminals by means ofconductors, wherein said device comprises a circuit for monitoring theintegrity of the conductors.
 23. Device for determining the weight of aperson sitting on a seat, comprising a means for determining therespective positions of the centres of gravity of the active weight inat least two different sections of the said seat and a means forevaluating the weight of the said person from the said respectivepositions so determined.
 24. Device according to claim 23, wherein thesaid means for determining the positions of the centres of gravitycomprises a means for detecting the distance between a first centre ofgravity of the weight on a first section of the seat and a second centreof gravity of the weight on a second section of the seat, the twosections of the seat being laterally adjacent.
 25. Device according toclaim 23, wherein said means for determining the position of the centresof gravity comprises a position-defining force detector which extendsover the surface of the seat.
 26. Device according to claim 25,characterised in that the said position-defining force detectorcomprises several strip-shaped active areas, the said active areas beinglocated on both sides of a line of separation of the said two sectionsand extending parallel to the said line.
 27. Device according to claim26, wherein the different active areas of a section of the seat aresupplied with different voltages.
 28. Device according to claim 27,wherein each active area of the said first section of the seat and thecorresponding active area of the said second section of the seat aresupplied with the same voltage.
 29. Device according to claim 26,wherein the active areas of a section are supplied by means of apotential gradient through several resistors connected in series, sothat the circuitry of the active areas represents a linear potentiometercircuit.
 30. Device according to claim 26, wherein each active areacomprises several individual sensors which are placed in line in thelongitudinal direction of the seat.
 31. Device according to claim 30,wherein the individual sensors of each active area are supplied withdifferent voltages.
 32. Device according to claim 31, wherein thecorresponding individual sensors of the different active areas of asection are supplied with the same voltage.
 33. Device according toclaim 31, wherein the individual sensors of a section are supplied bymeans of a potential gradient through a linear resistor, so that thecircuitry of the individual sensors represents a linear potentiometercircuit.
 34. Device according to claim 25, wherein the said forcedetector comprises force sensors whose electric resistance varies withthe applied force.
 35. Device according to claim 26, wherein said activeareas are connected to connection terminals by means of conductors,wherein said device comprises a circuit for monitoring the integrity ofthe conductors.
 36. Device according to claim 23, wherein the said meansfor determining the positions of the centres of gravity is incorporatedin the cushion of the seat.
 37. Method for determining the weight of aperson sitting on a seat, comprising the steps: subdivide the surface ofthe seat into at least two sections, determine the respective positionof the centre of gravity of the active weight in each section, andevaluate the weight of the said person from the said respectivepositions so determined.
 38. Method according to claim 37, wherein thesurface of the seat is subdivided into two laterally adjacent sections,and wherein the evaluation of the weight comprises the determination ofthe distance between the positions of the two centres of gravity of theweight in the said two sections.
 39. Method according to claim 37,comprising the additional step of evaluating the position of the saidperson on the said seat from the distribution of the positions of thecentres of gravity on the seat.
 40. Method according to claim 37,comprising the additional step of evaluating the orientation of the saidperson on the said seat from the longitudinal positions of the centresof gravity of the active weight in each section.
 41. Method according toone of claim 39 or 40, wherein the evaluation of the differentparameters is carried out sequentially.
 42. Use of a device according toclaim 23 in the control of an airbag of a motor vehicle.
 43. Deviceaccording to claim 31, wherein the individual sensors of a section aresupplied by means of a potential gradient through several resistorsconnected in series, so that the circuitry of the individual sensorsrepresents a linear potentiometer circuit.
 44. Device according to claim30, wherein said individual sensors are connected to connectionterminals by means of conductors, wherein said device comprises acircuit for monitoring the integrity of the conductors.
 45. Device fordetermining the size and the weight of a person sitting on a seat,comprising a means for determining the respective positions of thecentres of gravity of the active weight in at least two differentsections of the said seat and a means for evaluating the size and theweight of the said person from the said respective positions sodetermined.
 46. Method for determining the size and the weight of aperson sitting on a seat, comprising the steps: subdivide the surface ofthe seat into at least two sections, determine the respective positionof the centre of gravity of the active weight in each section, andevaluate the size and the weight of the said person from the saidrespective positions so determined.
 47. Method according to claim 46,wherein the surface of the seat is subdivided into two laterallyadjacent sections, and in that the evaluation of the size and the weightcomprises the determination of the distance between the positions of thetwo centres of gravity of the weight in the said two sections. 48.Method according to claim 46, comprising the additional step ofevaluating the position of the said person on the said seat from thedistribution of the positions of the centres of gravity on the seat. 49.Method according to claim 46, comprising the additional step ofevaluating the orientation of the said person on the said seat from thelongitudinal positions of the centres of gravity of the active weight ineach section.